The invention relates to a biaxially oriented opaque polyolefin multi-layer film which can be sealed on both sides and comprises a base layer and two top layers. The base layer is composed essentially of a propylene polymer with the addition of an inorganic pigment. The invention also relates to a process for the production of the film and to its use.
For many applications, particularly for packaging food, sufficiently opaque, grease-repellent and water-vapor proof films are required.
In high-speed packaging machines such as, for example, horizontal forming, filling and sealing machines and, in particular, also for packaging chocolate bars, films are required which, on the one hand, can be sealed on both sides and, on the other hand, have good slip properties in high-speed packaging machines and additionally show good printability, even after prolonged storage of the film.
A biaxially oriented polyolefin film which can be sealed on both sides and has three layers is disclosed in U.S. Pat. No. 4,419,411. In the case of this film, an additive combination is incorporated in both of the sealing layers. They each contain 0.15% to 0.4% by weight of polysiloxane and 0.05% to 0.5% by weight of silicon dioxide, the percentages by weight relating in each case to the sealable layer. The base layer is composed of polypropylene (as the main component) and contains a small amount of a monocarboxamide. Part of the amide migrates from the base layer into the two sealing layers and onto the surfaces thereof (external faces), so that polysiloxane, silicon dioxide and the monocarboxamide are present in incorporated form in each sealing layer, and the monocarboxamide is also present on the outer faces of the two sealing layers. The polyolefin multi-layer film described is stated to have a particularly low coefficient of friction. However, this film suffers in particular from the disadvantage that it is not printable.
In European Patent No. 0,004,633 an opaque multi-layer film is described, which comprises a polypropylene base layer modified with 5% to 15% by weight of a pigment and at least one sealing layer of a random C.sub.2 /C.sub.3 copolymer. During stretch orientation the polymer matrix tears open at the grain boundaries of the pigments; the hollow spaces, so-called vacuoles, thus produced cause a considerable reduction of film density and, as a consequence, opacity which imparts an attractive pearlescent appearance to the entire film.
To improve processability, the base layer of this multi-layer film can also contain migrating lubricants according to U.S. Pat. No. 4,419,411, for example, fatty acid esters or fatty acid amides, which accumulate at the film surface to form a lubricating film. The processing behavior of the film in the above-described high-speed packaging machines is increasingly improved upon prolonged storage, due to the formation of this lubricating film, at the same time, however, the adherence of printing ink is reduced to a considerable extent after a storage time of only 4 weeks.